It is generally known that good electric signal transmission properties in flat cables can be achieved by using for the dielectric a material having a low dielectric constant and low dielectric loss with little dependency on signal frequency.
The lower the dielectric constant of the dielectric, the more compact it is in size and the faster the signal propagation speed, if the characteristic impedance of the flat cable is the same. The smaller the dielectric loss, the smaller the signal dissipation becomes. The less frequency dependence of the dielectric constant and dielectric loss (e.g., pulse signals are caused to transmit through the flat cable) the smaller the pulse deformation is suppressed.
It is, therefore, required that the dielectric used in the flat cable have a low dielectric constant and low dielectric loss with little dependency on signal frequency. As one of such dielectric materials, expanded, porous polytetrafluoroethylene (PTFE) having a microstructure of numerous fine nodes interconnected by fine fibrils with continuous microscopic pores between the nodes and fibrils has been used. The expanded porous PTFE is, however, poor in dimensional stability since its texture is too soft. Thus, it is unsuitable as a dielectric of a flat cable, although it is used as a coaxial cable dielectric wrapped as a tape around a center conductor.
Furthermore, if a flat cable is made with the expanded porous PTFE alone as its dielectric, the spacing between conductors is dimensionally unstable and the stripping of the terminal dielectric material for connection is difficult, hence the flat cable so produced is almost impractical.